Electrodes for generating a stable discharge in gas laser systems

ABSTRACT

Arcing is minimized in a discharge chamber of a gas laser system by utilizing an electrode which comprises a surface portion capable of functioning as one of an anode and a cathode in order to energize a gas mixture in a discharge chamber of the gas discharge laser system, a shoulder portion being positioned on either side of the surface portion and being exposed to the gas mixture, and a coating layer made of electrically insulating material, wherein the coating layer is attached to the shoulder portion by a cold spraying method.

PRIORITY

This application is a continuation of U.S. patent application Ser. No.11/711,193 (now U.S. Pat. No. 7,756,184) filed Feb. 27, 2007.

TECHNICAL FIELD

The subject invention relates to improved electrodes for use in gaslasers such as excimer lasers.

BACKGROUND

In a conventional discharge chamber of a gas laser system, a pair ofelectrodes connected to a discharge circuit imparts electrical energy toa gas mixture volume located between the electrodes. The energizing ofthe gas mixture volume results in an excitation of the atoms and/ormolecules in the gas mixture volume. The atoms and/or molecules remainin the excited state only for a very short period of time, i.e., roughly10⁻⁸ seconds and a stimulated emission takes place if certainprerequisites are fulfilled. The stimulated emission leads to thegeneration of coherent radiation. In order to obtain a highlydirectionally orientated light beam, the discharge chamber is positionedwithin a resonator which is an optical feedback system and which usuallycomprises two mirrors. The mirrors are arranged on opposite sides of theresonator, thus forcing the radiation to oscillate within the resonator.One of the mirrors is totally reflective while the other allows afraction of the light to escape from the resonator, e.g., through apartially transparent section, thereby forming a laser beam.

Many gas laser systems produce short excitation pulses, leading to laserpulses of 10 ns to 30 ns. For improving the spatial uniformity of thegas discharge, the gas mixture volume is preionized by using, forexample, preionization pins. The preionization pins are placed close tothe electrodes and generate a spark discharge some 10 ns before the maingas discharge. The sparks produce ultraviolet radiation which issufficient to preionize the gas mixture volume between the electrodeswith a homogeneous initial seed density of about 10⁸ electrons/cm³.

Many applications, for example optical microlithography for formingsmall electronic structures on silicon substrates, require gas lasersystems to be run at high power, while maintaining a necessarily highrepetition rate of the laser pulses. This is achieved by utilizingdischarge chambers having a compact design. Compactly designed dischargechambers, however, tend to promote arcing which is not suitable for thegeneration of a laser beam and which damages the discharge chambers'electrodes.

Usually, the electrodes have a conductive structure comprising anextending ridge (sometimes referred to as a “nose portion”) and opposedshoulder portions. The protruding ridge portion is used to maintain theappropriate gap distance between the cathode and anode electrodes and toseparate the bulk of the electrode bodies from each other. Thisseparation can help to prevent arcing between, for example, shoulderportions of opposing electrodes, as well as from preionization pins toshoulder regions of the electrodes.

However, the use of a protruding ridge portion cannot guarantee theabsence of arcing. Additionally, the protruding ridge portion can have astrong influence on the flows of the gas mixture between the electrodes.An optimized gas flow with higher gas speeds, such as on the order ofabout 30-50 m/s, can be necessary for a high repetition rate laser over4 kHz. Due to unavoidable chemical reactions between the gas mixture andmaterials of the gas laser system as well as electrode burn-off, bothcausing solid and gaseous impurities, the gas mixture must becontinuously cleaned by special gas filters. Furthermore, an optimizedgas flow is necessary to lead away the excess heat produced during thelaser beam generation process.

An improved approach that can be used to avoid arcing, while improvingthe gas flow between the electrodes, is disclosed in U.S. Pat. No.7,079,565, assigned to the common assignee of this application. Asdescribed in this patent, a ceramic spoiler is placed at each shoulderregion of an electrode, to act as an insulating barrier over theshoulder portion of the electrode. The outer surface of each ceramicspoiler has a shape that is optimized to improve the flow of gas betweenthe electrodes. For mounting a ceramic spoiler to the electrode, theceramic spoiler comprises a projecting tongue which is received in achannel of the electrode. The projecting tongue is biased against thesides of the channel by a spring mounted in the channel so that theceramic spoiler is held in a stationary position with respect to theelectrode.

One drawback to the use of ceramic spoilers of this type is that theceramic spoilers have a certain minimal thickness due to themanufacturing process and mounts needed to connect the spoilers to theelectrodes. Thus, the necessary minimal thickness of the ceramicspoilers sets a minimal limit for the dimensions of the dischargechamber. Furthermore, the necessity for a mount increases manufactureand material costs.

Accordingly, it would be desirable and highly advantageous to have anavenue to minimize the occurrence of arcing in a discharge chamber of agas discharge laser system, and yet not to set a minimal limit for thedischarge chambers' dimensions.

Therefore, an aspect of the present invention is to provide an electrodefor a gas discharge laser system which minimizes the occurrence ofarcing in the discharge chamber while not limiting the reduction of thedischarge chambers' size.

A further aspect of the present invention is to provide an electrode fora gas discharge laser system which involves lower manufacture andmaterial costs than an electrode having separate ceramic spoilers.

A still further aspect of the present invention is to provide anelectrode for a gas discharge laser system which improves thepreionization of the gas mixture volume and which is less subject toerosion, thereby allowing for a better long-term stability of the gasdischarge.

SUMMARY OF THE INVENTION

In accordance with these and other objects, an electrode assembly for agas laser is disclosed which includes a pair of opposed electrodesarranged in face to face relationship leaving a gap therebetween. Inoperation, electrical power is applied to the electrodes cause a gasdischarge to be struck in the gap.

In accordance with the subject invention, at least one of saidelectrodes is partially coated with a non-porous ceramic material layer.Preferably, the layer is relatively thin, having a thickness less than 1mm.

In one preferred embodiment, the electrodes have a generallyhemispherical configuration, with a narrow central surface portion andopposed shoulder portions flanking the central surface portion. In thisconfiguration, the ceramic material coating layer is applied to theshoulder portions. In one embodiment, the central surface portion israised to define a ridge or nose. In this case, it is preferably tomatch the thickness of the ceramic layer to the height of the ridge sothat electrode has a smooth surface.

In another aspect of the subject invention, the central surface portionis provided with an average roughness of 100 microns. Roughening thesurface portion increases the surface area of the active electroderegion improving performance.

For a more complete understanding of the present invention and itsfeatures and advantages, reference is made to the following description,taken in conjunction with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically a cross-sectional view of a prior artdischarge chamber,

FIG. 2 illustrates schematically the surface regions of a dischargechamber according to an embodiment of the present invention which arepreferably coated with an insulating material,

FIG. 3 illustrates schematically a cross-sectional view of an electrodeaccording to an embodiment of the present invention having a ridge ornose portion, and

FIG. 4 illustrates schematically a cross-sectional view of an electrodeaccording to an embodiment of the present invention without a noseportion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates schematically a cross-sectional view of a prior artdischarge chamber. The drawing shows an upper and a lower electrode.Between the opposite electrodes the volume in which the gas dischargetakes place is depicted. On each side of the upper electrode, apreionization pin is shown. The dashed lines illustrate typical forms ofarcing which may occur in a prior art discharge chamber. As is sketchedin the drawing, arcing may occur between shoulder portions of oppositeelectrodes as well as between a preionization pin and the opposingelectrode's shoulder portion.

FIG. 2 illustrates schematically the surface regions of a dischargechamber according to an embodiment of the present invention which arepreferably coated with an insulating material. The electricallyinsulating coating layers are schematically depicted by bold lines. Itwill be apparent to a person skilled in the pertinent art that eachcoating layer is completely attached to the respective surface region.Besides the shoulder portions of the electrodes, it is also possible tocoat the surfaces of the preionization pins so that arcing between apreionization pin and the opposing electrode's shoulder portion isprevented while preionization of the gas mixture is still possible.

The coating layer made of electrically insulating material acts as aninsulating barrier over the shoulder portion of the electrode so that noarcing can occur on the shoulder portion. The thickness of the coatinglayer is preferably less than about 1 mm, and more preferably about 0.5mm. Therefore, the thickness of the coating layer is negligible withrespect to the dimensions of the electrode so that the coating layerdoes not set a limit for the reduction of the electrode's size, and thusthe size of the discharge chamber.

The electrically insulating material is preferably a ceramic material.Ceramic materials are capable of resisting the aggressive gasescontained in a gas mixture of a gas discharge laser system. Preferably,the ceramic material is aluminium(III) oxide. However, other ceramicmaterials may also be used, for example, zirconium(IV) oxide.

The ceramic material is preferably coated onto the shoulder portion byusing a cold spraying method, such as is available from CGT—Cold GasTechnology GmbH with offices in D-84539 Ampfing, Germany, and which isdescribed in the article “Beschichtungs-Technologie mit neuenMöglichkeiten” published in Konstruktion, Apr. 4, 2003.

An electrode having a coating layer made of ceramic material isconsiderably less complex in structure and manufacture than an electrodewith ceramic spoilers so that manufacture and material costs arereduced. Especially, the use of a cold spraying method simplifies thecoating process and lowers the costs. Furthermore, a cold sprayingmethod allows for a precise control of the areas of the electrode'ssurface which are to be coated, and also sets fewer restrictions due tothe coating process on the selection of the areas.

FIG. 3 illustrates schematically a cross-sectional view of an electrodeaccording to an embodiment of the present invention. In this embodimentof the present invention, the electrode includes a conductive structurecomprising a ridge or nose portion 2 and a shoulder portion 3. The noseportion 2 runs the entire length of the electrode, or a portion of theelectrode's length. For gas discharge laser systems with repetitionrates of 1-6 kHz, the nose portion 2 preferably is on the order of 1-4mm in width and 2-4 mm in height. Gas discharge laser systems withrepetition rates of 6 kHz or higher preferably use electrodes having anose portion 2 the width of which is on the order of 1 mm or lower, witha height on the order of about 2 mm.

The shoulder portions 3 are positioned on either side of the noseportion 2. An electrically insulating coating layer 5 is coated onto theelectrode's shoulder portion 3. Next to the nose portion 2, thethickness of the coating layer 5 is substantially equal to the height ofthe nose portion 2 so that the coating layer 5 is approximately flushwith the upper surface of the nose portion 2. The upper surface of thenose portion 2 is bounded by the coating layer 5 and forms a surfaceportion 8. The surface portion 8 is capable of functioning as one of ananode and a cathode in order to energize a gas mixture in a dischargechamber of a gas discharge laser system. An advantage of this embodimentof the present invention is the fact that the flow of the gas mixturebetween opposing electrodes is not negatively influenced byirregularities in the electrode's surface. Furthermore, an electrodeaccording to this embodiment of the present invention shows an improvedresistance to temporal changes of its shape and size, which leads to abetter long-term stability of the gas discharge.

The surface portion 8 is preferably treated by sandblasting with jets ofsteel grit to obtain an average surface roughness of substantially atleast 100 μm, whereby the energizing of the gas mixture is improved.

FIG. 4 illustrates schematically a cross-sectional view of an electrodeaccording to a preferred embodiment of the present invention without anose portion. In this embodiment of the present invention, theelectrode's surface is coated with an electrically insulating coatinglayer 5. At the apex of the electrode, the coating layer 5 comprises agap leaving a surface portion 8 of the surface of the electrode'sconductive structure uncoated. The surface portion 8 is capable offunctioning as one of an anode and a cathode in order to energize a gasmixture in a discharge chamber of a gas discharge laser system. Thethickness of the coating layer 5 is preferably less than about 0.5 mm atthe bounds of the surface portion 8 and increases to about 0.5 mm to 1mm in the region of the electrode's shoulder portion 3. An advantage ofthis embodiment of the present invention is its simple geometry. Theabsence of a nose portion simplifies the manufacture of an electrodeaccording to this embodiment of the present invention which contributesto cost effectiveness. The region of the electrode's surface which formsthe surface portion 8 is preferably treated by sandblasting with jets ofsteel grit to obtain an average surface roughness of substantially atleast 100 μm, whereby the energizing of the gas mixture is improved.Furthermore, an electrode according to this embodiment of the presentinvention shows an improved resistance to temporal changes of its shapeand size, which leads to a better long-term stability of the gasdischarge.

Surface Roughness of Electrodes

In the article of C. Yamabe, T. Matsuchita, S. Sato, and K. Horii,published in J. Appl. Phys., 1980, Vol. 51, N2, p. 898, it is shown that80% of the initial electrons of a gas discharge in a gas discharge lasersystem, which are provided by preionization, result from thephotoelectric effect at the surface of the electrode.

A skilled person in knowledge of the present invention will appreciatethat enlargement of the electrode's surface increases the number of theinitial electrons, and thus improves the energizing of the gas mixture.While leaving the shape and size of the electrode untouched, anenlargement of the electrode's surface is achieved in accordance with anembodiment of the present invention by increasing the surface roughness.An average surface roughness of substantially at least 100 μm has beenproven to be best.

For attaining a surface roughness of substantially at least 100 μm, thesurface portion is preferably treated by sandblasting with jets of steelgrit. Alternatively, the surface portion may be subject to a chemicaletching process.

A further advantage of an increased surface roughness is the effect thatthe surface comprises tiny peaks each of which having a higherelectrical field than a smooth surface has. The increased electricalfield of the peaks results in a reduced breakdown voltage of thesurface, i.e., the surface portion, so that possible arcing primarilyoccurs on this portion of the electrode's surface. The concentration ofpossible arcing on the surface portion, thus, leads to a reducedtemporal change of the electrode's shape and size, since the surfacestructure of a surface with higher surface roughness is less influencedby, for example, breaking out of material due to electrode burn-off thanthe surface structure of a smooth surface. Hence, an electrode accordingto this embodiment of the present invention shows an improved resistanceto temporal changes of its shape and size, which leads to a betterlong-term stability of the gas discharge.

An electrode according to an embodiment of the present invention ispreferably used in a gas laser system, comprising a resonator includingtherein a discharge chamber filled with a gas mixture, and a pair ofelectrodes in the discharge chamber and connected to a discharge circuitfor energizing the gas mixture and generating a laser pulse.

Using electrodes according to an embodiment of the present invention ina gas laser system allows for building a compactly designed dischargechamber, and thus a gas laser system having high repetition rates andimproved laser beam parameters, such as, for example, pulse energy, andpulse-to-pulse stability. Furthermore, the lifetime of the gas lasersystem is increased.

It should be recognized that a number of variations of theabove-identified embodiments will be obvious to one of ordinary skill inthe art in view of the foregoing description. Accordingly, the inventionis not to be limited by those specific embodiments and methods of thepresent invention shown and described herein. Rather, the scope of theinvention is to be defined by the following claims and theirequivalents.

1. A method of forming an electrode for exciting a gas in an excimerlaser to create a discharge comprising the steps of: providing anelongated metal electrode where the surface exposed to the gas dischargehas a hemispherical configuration in cross section and wherein thehemispherical surface is defined by a continuous smooth arc; and spraycoating a thin layer of non-porous ceramic material over the entirehemispherical surface of the electrode except for a narrow centralregion at the center of the hemispherical surface and wherein thethickness of the layer is less than 1 mm.
 2. A method as recited inclaim 1, wherein the thickness of the coating increases from the centralregion to the outer side edges of the hemispherical surface.
 3. A methodas recited in claim 1, further including the step of roughening thecentral region to achieve an average surface roughness of about 100microns.
 4. A method as recited in claim 3, wherein said roughening isperformed by sandblasting the central region by a jet of steel grit. 5.A method as recited in claim 1, wherein the ceramic material is aluminum(III) oxide.
 6. A method as recited in claim 1 wherein the thin layer issprayed on the electrode with a cold spraying technique.